[mythtv-users] Resolution

papenfuss at juneau.me.vt.edu papenfuss at juneau.me.vt.edu
Wed Dec 3 15:30:49 EST 2003


On Wed, 3 Dec 2003,  James L. Paul wrote:
> > SO, with this in mind, take a few data points.  NTSC VHS has 240 lines.
> > Formatted at 4:3, this translates into 480 vertical pixels (NTSC viewable
> > scan lines), and horizonally (240 * 4/3) = 320.  So, to capture VHS-quality
> > video, you need a capture resolution of 320x480... 1/2 D1 is close at
> > 352x480.  To capture high-quality broadcast at 330 lines, (330 * 4/3) =
> > 440... SVCD is close at 480x480.  Capturing any more than that wastes
> > encoding time, space, encoding quality, etc.
> 
> This is a much better description than mine. As I remember it though there was 
> some effect caused by interlacing, real-world signal loss/distortion and 
> quality of video tape that made VHS generally considered 352x240, although I 
> can see now the 352 is obviously inflated. (Perhaps that explains why all the 
> VCDs I made from my DirecTV looked better than VHS from the same source.)
> I think it's the way the interlaced frames are helically laid down on the 
> tape, there is significant bleed on VHS, but SVHS used denser tape and 
> probably smaller heads.

	The 352 is not all that inflated.  At the rated 240 lines, that equates 
to 320 pixels horizonally.
> 
> So the way I understood it, although all NTSC video has 525 lines, about 480 
> of them viewable, the analog storage and reproduction of VHS tape doesn't 
> accurately retain the resolution of all them, and results in about half. It's 
> as if the interlaced video interpolates itself unnecessarily. This may be 
> totally wrong. I've never questioned it though since seems to match my 
> experience.
> 
	I thought about that, and that's actually what I was thinkign when I 
was searching for the definition of resolution.  I'd be tempted to believe it, 
but remember that two adjacent lines on the screen are put on the tape 1/30th 
of a second later (well, 1/29.97th, but who's counting!).  If they were to 
bleed into each other, they would soften the whole thing up so that you 
couldn't distinguish individual fields.  Recording from VHS tape at 480 
vertical pixels definately shows a clearly interlaced picture.

	I'm not saying that there's not bleed-through between adjacent scans on 
the tape, but it's not much of a factor.

-Cory

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